MRI-Safe and Force-Optimized Implantable Ring Magnet System with an Enhanced Inductive Link

ABSTRACT

A magnetic system for a medical implant system is described. A planar implant receiver coil is configured to lie underneath and parallel to overlying skin of an implanted patient for transcutaneous communication of an implant communications signal. A planar ring-shaped attachment magnet also is configured to lie underneath and parallel to the overlying skin and radially surrounds the receiver coil. The attachment magnet is characterized by a magnetic field configured to avoid creating torque on the attachment magnet in the presence of an external magnetic field.

This application claims priority from U.S. Provisional PatentApplication 62/540,117, filed Aug. 2, 2017, which is incorporated hereinby reference in its entirety.

FIELD OF THE INVENTION

The present invention relates to medical implants, and more specificallyto a permanent magnet arrangement for use in such implants.

BACKGROUND ART

Some implants such as for example Vestibular Implants, (VI's), MiddleEar Implants (MEI's) and Cochlear Implants (CI's) employ attachmentmagnets in the implantable part and an external part to hold theexternal part magnetically in place over the implant. For example, asshown in FIG. 1, a typical cochlear implant system may include anexternal transmitter housing 101 containing transmitting coils 107 andan external attachment magnet 105. The external attachment magnet 105has a conventional coin-shape and a north-south magnetic dipole that isperpendicular to the skin of the patient to produce external magneticfield lines M₁ as shown. Implanted under the patient's skin is acorresponding receiver assembly 102 having similar receiving coils 108and an internal attachment magnet 106. The internal attachment magnet106 also has a coin-shape and a north-south magnetic dipole that isperpendicular to the skin of the patient to produce internal magneticfield lines M₂ as shown. The internal receiver housing 102 is surgicallyimplanted and fixed in place within the patient's body. The externaltransmitter housing 101 is placed in proper position over the skincovering the internal receiver assembly 102 and held in place byinteraction between the internal magnetic field lines M₂ and theexternal magnetic field lines M₁. Rf signals from the transmitter coils107 couple data and/or power to the receiving coil 108 which is incommunication with an implanted processor module (not shown).

One problem arises when the patient undergoes Magnetic Resonance Imaging(MRI) examination. Interactions occur between the implant magnet and theapplied external magnetic field {right arrow over (B)} of the MRI. Asshown in FIG. 2, the direction of the magnetic dipole moment {rightarrow over (m)} of the implant attachment magnet 202 is perpendicular tothe skin of the patient. Thus, the external magnetic field {right arrowover (B)} from the MRI may create a torque {right arrow over (T)}={rightarrow over (m)}×{right arrow over (B)} on the attachment magnet 202,which may displace the attachment magnet 202 or the whole implanthousing 201 out of proper position. Among other things, this may damagethe adjacent tissue in the patient. In addition, the external magneticfield {right arrow over (B)} from the MRI may reduce, remove or invertthe magnetic dipole moment {right arrow over (m)} of the attachmentmagnet 202 so that it may no longer be strong enough to hold theexternal transmitter housing in proper position. The attachment magnet202 may also cause imaging artifacts in the MRI image, as well as thereare maybe induced voltages in the receiving coil creating hearingartifacts. This is especially an issue with MRI field strengthsexceeding 1.5 Tesla.

Thus, for existing implant systems with magnet arrangements, it iscommon to either not permit MRI, or at most limit use of MRI to lowerfield strengths. Other existing solutions include use of surgicallyremovable attachment magnets, spherical attachment magnets (e.g. U.S.Pat. No. 7,566,296), and various ring magnet designs (e.g., U.S. Pat.No. 8,634,909 and U.S. Patent Publication 2011/0022120), all of whichare incorporated herein by reference. Various other complex arrangementsof magnetic elements have been described for use in hearing implantapplications; See for example, U.S. Pat. Nos. 4,549,532 and 7,608,035,which are incorporated herein by reference. However, there is nosuggestion that such therapeutic arrangements might potentially have anyutility for magnetic attachment applications such as those describedabove.

SUMMARY OF THE INVENTION

Embodiments of the present invention are directed to a magnetic systemfor a medical implant system. As shown in FIG. 3 planar implant receivercoil 302 is configured to lie underneath and parallel to overlying skin307 of an implanted patient for transcutaneous communication of animplant communications signal. A planar ring-shaped attachment magnet301 is also configured to lie underneath and parallel to the overlyingskin 307 and radially surrounds the receiver coil. The attachment magnet307 is characterized by local magnetic dipole moments configured toavoid creating or at least minimize torque on the attachment magnet inthe presence of an external magnetic field. This can be achieved by aratable diametrical overall magnetization or a complex magnetizationwhere the integral of all local magnetic dipole moments is zero.

In further specific embodiments, there is an implant housing configuredto contain the implant receiver coil and the attachment magnet. Theimplant housing and the attachment magnet may be configured to enablerotation of the attachment magnet within the implant housing to avoidcreating torque on the attachment magnet in the presence of an externalmagnetic field. Or the implant housing and the attachment magnet may beconfigured to prevent rotation of the attachment magnet within theimplant housing. There may be a volume of damping oil within the implanthousing around the attachment magnet configured to resist movement ofthe implant magnet within the implant housing

The attachment magnet may include multiple local magnetic sections,wherein each domain can be either radially or diametrically magnetizedto enable a ring-magnet acting by connecting the single domains. Thesegmented magnet may have the same magnetic and MRI behavior as thedescribed ideal ring magnet and may be encapsulate in a biocompatiblematerial to guarantee mechanical and biological safety.

The attachment magnet may include multiple local magnetic sections,wherein each local magnetic field has an independent local magneticfield and an independent local magnetic field direction, and wherein acombined magnetic field for all the local magnetic fields has a zerooverall magnetic dipole moment. In such an embodiment, each localmagnetic section may be a physically distinct ring segment, whereby theattachment magnet comprises multiple ring segments connected together toform a ring shape. Or the attachment magnet may comprise a singleintegral structure without distinct ring segments. In anotherembodiment, the attachment magnet may be characterized by a singlemagnetic dipole moment oriented across the ring diameter parallel to theoverlying skin.

The receiver coil may have a disk shape. And the attachment magnet isencapsulated in biocompatible material.

In any of the above, the medical implant system may be a hearing implantsystem such as a cochlear implant system, a middle ear implant system, abone conduction hearing implant system, or a vestibular implant system.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a portion of a typical idealized cochlear implant which maybe used in embodiments of the present invention.

FIG. 2 shows effects of an external magnetic field on an implantedportion of an implanted part which may be used in embodiments of thepresent invention.

FIG. 3 shows a cross-sectional view of a coil and attachment magnetarrangement according to an embodiment of the present invention.

FIG. 4 shows a top plan view of an attachment magnet according to anembodiment of the present invention.

FIG. 5 shows a top plan view of an attachment magnet according toanother embodiment of the present invention.

FIG. 6 shows a top plan view of an attachment magnet according toanother embodiment of the present invention.

FIG. 7 shows a top plan view of an attachment magnet according toanother embodiment of the present invention.

FIGS. 8A and 8B show an attachment magnet according to anotherembodiment in relation to an external attachment magnet and the holdingforce therebetween as a function of the rotational angle, respectively.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

Various embodiments of the present invention are directed to an improvedimplant attachment magnet arrangement that reverses the conventionalstructure. Instead of a cylinder-shaped attachment magnet surrounded bya ring-shaped receiver coil, embodiments of the present invention have acenter receiver coil surrounded by a ring-shaped attachment magnet withone overall or locally varying magnetic dipole moment orientations. Oneadvantage of the invention is, that the ring shape of the attachmentmagnet distributes the attractive magnetic holding force exerted to theexternal attractive magnetic holding part to a larger area around thecenter coil, compared to the conventional design where the attractivemagnetic holding force is exerted to the external part from the smallcenter magnet to a small area around the coil center only. Anotheradvantage is, that the receiver and transmitter coils for transcutaneoustransmission may use the area up to the transmitter or receiver coilcenter, such that a better coupling can be achieved. This in turn canimprove in case of power transfer the power transfer efficiency and incase of data transfer the signal-to-noise ratio of the data signal atthe receiver coil. Another advantage of the ring-shaped attachmentmagnet is, that the volume and hence the holding force exerted to theexternal part can be increased in comparison to the conventional design.The ring-shaped attachment magnet also offers greater internal space forelectronic components, which can lead to a size reduced implant and acorrespondingly smaller external part size as well. The internal spacemay have one sidewall facing the bone or skin of the patient head thatmay be adapted to fit the outer shape of the bone or skin for theimplantable and external part respectively. For example, the sidewallmay have a concave shape. For the external part this may further improvethe attractive magnetic holding force distribution and therebyadditionally increase the retention abilities and patient comfort.

FIG. 3 shows a planar implant receiver coil 302 surrounded by thering-shaped attachment magnet 301, configured to be implantable in apatient underneath and parallel to the overlying skin 307 fortranscutaneous communication. The Communication signals are transmittedfrom a corresponding transmitter coil 304 in an external part 309 whichis held in place on the skin 307 by magnetic attraction by theattachment magnet 301. The implant receiver coil 302 and the attachmentmagnet 301 have a radially symmetric shape about a common center axis306. The outer diameter (dc) of the receiver coil 302 is smaller thanthe inner diameter (di) of the attachment magnet 301. In one preferredembodiment, the ratio of the inner diameter (di) of the attachmentmagnet 301 to outer diameter (dc) of the receiver coil 302 is in therange from 0.74 to 0.76. The attachment magnet 301 interacts with anexternal attachment magnet 303 with a matching magnetic field to createthe attachment force that holds the external part 309 securely in placeon the skin 307 with the transmitter coil 304 directly over the receivercoil 302 for optimal coupling of the communications signal (that is forexample generated in an external signal processor 311 that also may belocated in the external part 309 as shown, or it may be locatedseparately). The attachment magnet 301 may have any suitable magneticdipole moment described herein by way of reference to any of theexemplary figures.

The specific embodiment shown in FIG. 3 includes an implant housing 308that has an outer surface 312 that is configured to lie under andparallel to the skin 307 and contains the implant receiver coil 302 andthe attachment magnet 301. The implant attachment magnet 301 may becovered within a magnet housing 305 of a biocompatible material toguarantee biocompatibility and mechanical resistance. The implanthousing 308 also contains implant circuitry 310 that processes theimplant communications signal from the receiver coil 302 to extract apower component to power the implanted components, and/or a datacomponent that is processed into one or more stimulation signals forimplanted electrodes (not shown). In one embodiment when implanted, theskull facing sidewall may be adapted to fit the outer shape of the skullbone, e.g. being concave shaped. In other embodiments, though, there maynot be such an implant housing 308; for example, the receiver coil 302and the attachment magnet 301 may be encapsulated in resilient materialwith the implant circuitry 310 physically separate. The resilientmaterial may be silicone.

FIG. 4 shows a top plan view of another embodiment of an attachmentmagnet 301 according one aspect of the present invention, that ischaracterized by a single magnetic dipole moment 601 that is orientedacross the ring diameter, i.e. diametrical magnetization, parallel tothe overlying skin or parallel to the common center axis 306, i.e. axialmagnetization, and perpendicular to the overlying skin (not shown)either oriented away or toward. An external applied magnetic field{right arrow over (B)} from e.g. an MRI scanner may create a torque{right arrow over (T)} on the attachment magnet 301. In one example, forimproving MRI safety, the attachment magnet 301 may be fixated to theunderlying bone by any suitable fixation means. This may for example bescrews or pins as known in the art.

In another preferred embodiment, the attachment magnet 301 has amagnetic dipole moment 601 parallel to the skin and is rotatable aroundthe common center axis 306 to align with the strong external magneticfield {right arrow over (B)} from e.g. the MRI and thereby avoidcreating a torque {right arrow over (T)} about the common center axis306 of the attachment magnet 301. This avoids, when implanted, contactforce onto the skull and thus pain to the patient and may even preventdisplacement of the attachment magnet 301 and thereby improves MRIsafety. For this purpose, one or more sliding surfaces of the attachmentmagnet 301 may be covered by titanium or some other material to reducefrictional abrasion. In some embodiments, the attachment magnet 301 maybe rotatable located in the interior volume of the magnet housing 305.At least one part of the interior volume of the magnet housing 305 mayinclude a volume of damping oil or ferromagnetic domain(s) around theattachment magnet 301 in a configuration that resists unintendedmovements of the attachment magnet 301 within the magnet housing 305 dueto for example movement of the patient head. The magnet housing 305 maybe an integral part of implant housing 308, where the internal volume ofthe magnet housing 305 may be formed by a closed compartment in theimplant housing 308. For this embodiment with a rotatable magnet designthe magnet material must not resist high demagnetization fields insideof a strong external magnetic field, because the internal magneticdipole moment is aligned to the external field resulting in no magnetmaterial harm.

FIG. 5 shows a top plan view of an attachment magnet 301 with radialmagnetization of the planar ring-shaped magnet design, where themagnetic dipole moment orientation {right arrow over (m)} at each pointis directed to the rotation symmetry axis 306. The correspondingattachment magnet 301 of the external part 309 has a magnetic dipolemoment orientation {right arrow over (m)} in the opposite direction todevelop an attractive force, i.e. the magnetic dipole moment {rightarrow over (m)} at each point is directed away from the rotationsymmetry axis of the external attachment magnet 301. This radialsymmetric magnetization results in a net zero magnet dipole moment{right arrow over (m)} and as a consequence no torque {right arrow over(T)} on the attachment magnet 301 occurs when exposed to an externalmagnetic field {right arrow over (B)}, for example from a MRI scanner.In this embodiment, the magnet material used for the attachment magnet301 must have the resistance against demagnetization, because at leastone magnet domain is orientated opposite to the external magnetic field,for example from a MRI scanner.

FIG. 6 shows a top plan view of an attachment magnet 301 showing anembodiment in which each local magnetic section is a physically distinctring segment 401 whereby the attachment magnet 301 comprises multiplering segments 401 connected together to form a ring shape. In thisembodiment, each segment can either have a radial symmetric magneticdipole moment {right arrow over (m)} or a diametrical magnetic dipole{right arrow over (m)} or a combination of both. As in FIG. 5, thesymmetric magnetic arrangement results in a net zero magnetic dipolemoment {right arrow over (m)} and as a consequence no torque {rightarrow over (T)} on the attachment magnet 301 occurs when exposed to anexternal magnetic field {right arrow over (B)}. In this embodiment, themagnet material has to have the resistance against demagnetization,because at least one magnet segment 401 has a magnetic dipole moment{right arrow over (m)} orientated into opposite direction to theexternal magnetic field {right arrow over (B)}, for example from a MRIscanner.

FIG. 7 shows a top plan view of an attachment magnet 301 showing anembodiment in which there are multiple local magnetic sections with eachhaving an independent local magnetic dipole moment {right arrow over(m)} and an independent local magnetic dipole moment orientation (asshown by the adjacent arrows) that in this example is parallel to theplane of the overlying skin. In one embodiment, the local magneticsections and their local magnetic dipole moment {right arrow over (m)}may be arranged and orientated in such a way, that the exerted holdingforce on the external attachment magnet 303 has at least onedistinguished maximum at one rotational angle about the common centeraxis 306 relative to the attachment magnet 301. Such an exemplaryarrangement is shown in FIG. 7. This has the advantage, that theexternal part 309 is held in place on the skin 307 in a fixed relativerotational relation to the (implanted) implant housing 308. This is forexample important for Vestibular Implants where the movement sensors inthe external part must have a known orientation in relation to thepatient. In addition, the overall combined magnetic dipole moment forall the local magnetic dipole moments {right arrow over (m)} togethermay have a zero combined magnetic dipole moment (the vector sum of thelocal magnetic dipole moments {right arrow over (m)} as shown in FIG. 7vanishes, i.e. for each section i having local magnetic dipole moment{right arrow over (m)}_(i), the vector sum yields:

$\left. {{\sum\limits_{i}{\overset{\rightharpoonup}{m}}_{i}} = 0} \right)$

and as a consequence no torque {right arrow over (T)} on the attachmentmagnet 301 occurs when exposed to an external magnetic field {rightarrow over (B)}, for example from a MRI scanner.

The attachment magnet 301 shown in FIG. 7 illustrates an example wherethe local magnetic sections and their local magnetic dipole moment arearranged in such a way, that the combined magnetic dipole momentvanishes and simultaneously the exerted holding force on a correspondingexternal attachment magnet 303 has at least one distinguished maximum atone rotational angle about the common center axis 306 relative to theattachment magnet 301. FIG. 8A shows the attachment magnet 301 in aconfiguration as shown in FIG. 7 and corresponding external attachmentmagnet 303 rotated relative to each other by angle α. The exemplarilyshown four local magnetic segments 401 of the external attachment magnet303 have opposite magnetic orientation in relation to their counterpartlocal magnetic segment 401 of the attachment magnet 301 to obtainmaximal attractive force. FIG. 8B shows the attractive force betweenattachment magnet 301 and external attachment magnet 303 as a functionof the rotational angle α about the common center axis 306. The force isstrongest at one angle α_(o). In one embodiment, the attachment magnet301 may be fixated, in the same way as described with reference to FIGS.5 and 6 above. Similarly, the magnet material must be resistant againstdemagnetization, because at least one magnet sections has a magneticdipole moment {right arrow over (m)} orientated into opposite directionto the external magnetic field {right arrow over (B)}, for example froma MRI scanner.

In another embodiment, the attachment magnet 301 shown in FIGS. 7 and 8may be rotatable about the common center axis 306 in relation to theimplant housing 308, as shown in FIG. 4. In this configuration, theimplantable part may comprise a sensor for detecting the relativerotational angle of the attachment magnet 301 in relation to the implanthousing 308. The sensor may comprise a magnetic field sensor to measurethe magnetic field of the attachment magnet 301 and provide to theimplant circuitry 310. Implant circuit 310 may be configured tocalculate the relative rotational angle in relation to implant housing308 from the measurement. Alternatively, instead of the sensor, a springelement may be used to bring the attachment magnet 301 reversibly backto a pre-determined rotational angle in relation to the implant housing308.

Although various exemplary embodiments of the invention have beendisclosed, it should be apparent to those skilled in the art thatvarious changes and modifications can be made which will achieve some ofthe advantages of the invention without departing from the true scope ofthe invention. For example, the ring-shape although shown as a circularring, may have any suitable geometric form, for example and withoutlimitation a rectangular, quadratic, triangular, oval with or withoutrounded edges.

1. A magnetic system for a medical hearing implant system for a patientuser, the system comprising: an implant housing having: a ring-shapedimplant receiver coil configured to lie underneath and parallel tooverlying skin of an implanted patient for transcutaneous communicationof an implant communications signal; and a ring-shaped attachment magnetconfigured to lie underneath and parallel to the overlying skin andradially surrounding the receiver coil,
 2. The system according to claim1, wherein the implant receiver coil is planar.
 3. The system accordingto claim 1, wherein the ring-shaped attachment magnet is planar.
 4. Thesystem according to any of claims 1, wherein the attachment magnet andreceiver coil is circular or oval ring-shaped.
 5. The system accordingto claim 4, wherein the ratio of the inner diameter of the ring-shapedattachment magnet to the outer diameter of the ring-shaped receiver coilis in the range from 0.74 to 0.76.
 6. The system according to any ofclaims 1, wherein the attachment magnet is characterized by a singlemagnetic dipole moment.
 7. The system according to claim 6, wherein thedipole moment is oriented across the ring diameter parallel to theoverlying skin or parallel to the center axis of the ring-shapedattachment magnet and perpendicular to the overlying skin.
 8. The systemaccording to claim 1, wherein the attachment magnet comprises aplurality of local magnetic sections, wherein each local magneticsection has an independent local magnetic dipole and an independentlocal magnetic dipole orientation.
 9. The system according to claim 8,wherein the combined magnetic dipole for all the local magnetic sectionshas a zero overall magnetic dipole moment.
 10. The system according toclaim 8, wherein the independent local magnetic dipoles are orientedsuch, that the exerted attractive holding force to an external partcomprising a ring-shaped external attachment magnet where the attachmentmagnet and the external attachment magnet are aligned about a commoncenter axis, has at least one distinguished maximum at one rotationalangle about the common center axis relative to the attachment magnet.11. The system according to any claim 8, wherein each local magneticsection is a physically distinct ring segment, whereby the attachmentmagnet comprises a plurality of ring segments connected together to forma ring-shape.
 12. The system according to claim 1, wherein the implanthousing and the attachment magnet are configured to prevent rotation ofthe attachment magnet within the implant housing.
 13. The systemaccording to claim 1, wherein the implant housing and the attachmentmagnet are configured to enable rotation of the attachment magnet withinthe implant housing in the presence of an external magnetic field. 14.The system according claim 13, wherein the implant housing furthercomprising a sensor adapted for determining the relative angle ofrotation between the implant housing and the attachment magnet. 15.(canceled)